Three- or multi-level NPC and NPP power converters are widely used in many different power conversion applications such as, but not limited to wind and solar converters, power supplies, and variable speed drives, including high power medium voltage low speed machines and Doubly Fed Induction Machines (DFIM). An NPC converter typically includes three phase legs and a DC bus capacitor bank comprising at least two series connected DC bus capacitors. Each phase leg is composed of four series connected switches and each switch has an antiparallel freewheeling diode. Two additional diodes, so-called clamping diodes, are connected between the leg and a DC bus midpoint. The NPP converter topology is substantially similar to the NPC topology, but the output converter phases are connected to the DC bus midpoint by an internal bi-directional switch instead of the clamping diodes. Three-level NPC or NPP converters have many advantages, including good utilization of the semiconductor switches and low distortion of the output voltage.
In the 3-Level NPC and NPP converter topologies the splitting of the DC bus capacitor bank into two capacitor sub-banks provides an intermediate voltage step (midpoint or neutral point) for the 3 step modulation of the output phase voltages, i.e. via switching between the positive or negative DC bus rails and the capacitor midpoint. An inherent property of the NPC and NPP topologies, however, is that the resultant capacitor midpoint current, which ideally has a zero average, in addition to high switching ripples, contains a significant low frequency content or ripples. This low frequency midpoint current drives differences between voltages of the upper and lower DC bus capacitors causing capacitor midpoint voltage variations which may affect operation of the converter.
The capacitor midpoint current has a variable peak or rms value and waveform shape depending on the converter current and its phase shift with respect to phase voltage, i.e. the power factor. It is dominated by its fundamental frequency found at 3 times the converter output frequency and is thus often also called “3rd harmonic midpoint current injection”. In addition to the 3rd harmonic current injection, due to various system asymmetries, the capacitor midpoint current may have some small non-zero average DC value which is source of a drift of average value of the capacitor midpoint voltage.
In practical applications, two 3-level NPC or NPP converters are often connected in so called back-to-back topology to allow indirect AC/AC (grid frequency input/variable frequency output) conversion with intermediate DC link stage, i.e. an AC/DC/AC conversion. In this topology one of the converters is connected to the power grid-side and forms grid-side AC/DC conversion stage which is operated at constant frequency of typically 50 Hz/60 Hz to control the grid currents and exchange power between the grid and DC link. The other converter is connected to a load, such as a three-phase machine and forms the load- or machine-side DC/AC conversion stage to control the machine currents and exchange power between the DC bus and machine. To allow power transfer via the DC link, the grid-side and machine-side stages must be interconnected (back to back) via the positive and negative dc bus rails with or without interconnection of the capacitor midpoints. It may be more advantageous to interconnect the capacitor midpoints of the grid and machine-side converter stages to stiffen up the capacitor midpoint potential.
The midpoint current and associated dynamic voltage ripple and static drift of the midpoint voltage are source of several significant effects which have to be considered in the NPC converter design. These effects include an increased voltage and current stress of the DC bus capacitors, increased losses in the capacitors, and reduced capacitor life time. Excessive midpoint voltage variations may increase stress of semiconductor switching devices and may cause activation of DC brake choppers or protective overvoltage converter trips.
The capacitor midpoint voltage ripple is an extremely important issue in applications of the 3L NPC/NPP converters for stator current control of high power medium voltage low speed machines or rotor currents in Doubly Fed Induction Machines (DFIM). In these applications the machine/rotor side converter can be operated at nearly full modulation depths at very low nominal frequencies, e.g. 3 Hz-5 Hz. In such operational conditions the midpoint current produced by the machine-side converter, which is dominated by the 3rd harmonic component, has a relatively low frequency, e.g. 9 Hz-15 Hz. Therefore, the capacitor midpoint voltage ripple created by the machine-side converter can be an order of magnitude higher than that created by the grid-side converter operated at 50 Hz/60 Hz nominal frequency, i.e. a midpoint current injection at 150 Hz/160 Hz. In order to keep the capacitor midpoint voltage ripple or oscillations within a tolerable level in all operational conditions, there are two basic options: passive means by using large DC bus capacitors and/or active means based on direct or indirect midpoint current control.
In low frequency machine applications the passive control of the DC midpoint voltage ripple requires a substantial increase of the DC bus capacitance which has strong cost and space implications. In the back-to-back converter topologies the capacitor midpoint voltage ripple in the machine-side converter can be partially passively reduced if the capacitor midpoints of the grid- and machine-side converters are interconnected. But in critical low speed applications the residual midpoint voltage ripple may still be excessive and further increase of DC bus capacitance is normally required.
To reduce the size of the DC bus capacitors in low frequency applications it is extremely advantageous to maximize utilization of the active means to reduce the capacitor midpoint voltage ripple. For example, it is well known that it is possible to eliminate or reduce the capacitor midpoint current injection in 3-level NPC/NPP converters in an active way via injection of common mode voltage into converter voltage references. Such active compensation algorithms are typically designed to control common mode voltage inserted into the converter voltage references with the goal to stabilize static drift of the average value of the midpoint capacitor voltage and to reduce its ripple which is associated with the converter own midpoint current injection. The static midpoint voltage drift and the converter midpoint current ripple can be fully compensated in this way only in the operational points where the power factor is high and when a sufficient modulation margin is available. Unfortunately in the most critical operational points of the machine-side converter the modulation margin may be relatively low so that the midpoint current injected by the machine-side converter may only be marginally compensated.
The most direct way to control the DC bus capacitor midpoint potential is to connect the capacitor midpoint to the supply neutral point, e.g. via a direct connection, as disclosed in U.S. Pat. No. 7,528,505 B2, for example. The advantage of this solutions is that a very effective capacitor mid-point voltage control can be achieved in practically all operational points. However, this solution relies on passive mechanisms and is effective mainly in prevention of drifts of average value of the capacitor midpoint voltage. Moreover, as the neutral current path is lightly damped, this can potentially lead to instabilities. A disadvantage of this solution is also that the converter input inductor must not be a three-phase magnetically coupled inductor. The input inductor must be composed of three independent single-phase inductors.
Alternatively, if a three-phase magnetically coupled inductor is used an additional inductor has to be inserted between the grid and converter neutral points. An example of such solution is presented in US 2013/0163292 A1, where the DC bus midpoint is connected to a grid transformer neutral via an inductor and an additional active control of the neutral point current is used to actively control the capacitor midpoint voltage. But regardless of effectiveness of both solutions, major disadvantages are that additional passive components and access to the grid transformer neutral point are required.
The midpoint voltage can also be controlled in an active way using the converter Pulse Width Modulation (PWM). The basic advantage of this solution is that there is no need for additional passive components. A common approach is to utilize controlled injection of the common mode voltage into the converter voltage references. In this way it is possible to alter the midpoint current injected by the converter without affecting the converter line-line voltages and introducing unwanted distortion into the converter phase currents. From U.S. Pat. No. 8,441,820 B2, for example, it is known that it is possible to find bilateral functional relationships between the converter midpoint current and the injected common mode voltage. This allows to shape the common mode voltage injection to control instantaneous value of the midpoint current injection for capacitor midpoint voltage control.
In the state of the art solutions, injection of the common mode voltage is performed with the goal to compensate either average drift and/or ripple in the midpoint voltage produced by the converter itself. U.S. Pat. No. 9,071,084 B2 discloses a back-to-back 3-level NPC converter with interconnected capacitor midpoint, where control of average midpoint voltage drift is shared between the grid- and machine-side converters depending on available modulation margins of the converters. Compensation of low frequency ripples caused by the machine-side converter is not achieved and not intended.
It is an object of embodiments of the invention to improve performance of the capacitor midpoint variation compensation in back-to-back 3-level converter topologies with interconnected capacitor midpoints in low frequency applications. In particular, it is an object of embodiments of the present invention to provide a control method and system for such converter topologies, which greatly facilitate compensation of capacitor midpoint variations, including the dominating 3rd harmonic ripples caused by the machine-side converter. Another object of embodiments of the invention is to provide a power conversion system based on back-to-back 3-level converter topologies with interconnected capacitor midpoints and having capabilities for efficient compensation of capacitor midpoint drifts and low frequency ripples.
To solve this object, embodiments of the present invention provide the control method having the features of independent claim 1, the control system of claim 11 and the power conversion system of claim 15. Embodiments of the present invention are subject-matter of the dependent claims.